1. Field of the Invention
This invention relates to a method for the reduction of polymer fouling in the fractional distillation of light end hydrocarbon components such as those produced by steam cracking. More particularly, the invention relates to a method of reducing fouling by use of a dual pressure, dual column fractionation configuration instead of a conventional single pressure, single column configuration.
2. Description of the Prior Art
Reaction conditions for steam cracking are selected to maximize the production of light olefins. Typically, cracking is practiced at a weight ratio of 0.3:1.0 of steam to hydrocarbon with the reactor coil outlet at 760.degree.-870.degree. C., and slightly above 100 kPa (atmospheric) pressure.
The type of feedstocks and the reaction conditions determine the mix of products produced. Many steam crackers operate on light paraffin feeds consisting of ethane and propane and the like. However, a significant amount of steam cracking capacity operates on feedstocks which contain propane and heavier compounds. Steam cracking such feedstocks produces many marketable products, notably propylene, isobutylene, butadiene, amylene and pyrolytic gasoline.
In addition to the foregoing, small quantities of undesirable contaminants, such as di- and polyolefins, and acetylinic compounds are produced. These contaminants may cause equipment fouling, interfere with polymerization reactions, and in some cases pose safety hazards. It is, therefore, highly desirable to remove them from the distillation stream. It is in the removable form the distillation stream of these contaminants that this invention has its application.
During steam cracking, cracked gases emerging from the reactors are rapidly quenched to arrest undesirable secondary reactions which tend to destroy light olefins. The cooled gases are subsequently compressed and separated to recover the various olefins.
The recovery of the various olefin products is usually carried out by fractional distillation using a series of distillation steps or columns to separate out the various components. The unit which separates the methane fraction (C.sub.1) is referred to as the "demethanizer," the unit which separates the ethane fraction (C.sub.2) is referred to as the "deethanizer," the unit which separates the propane fraction (C.sub.3) is referred to as the "depropanizer," and the unit which separates the butane fraction (C.sub.4) is referred to as the "debutanizer." The residual higher carbon number fraction (C.sub.5+) is used as gasoline.
With the development of selective furnace designs for very high conversion of liquid petroleum gas by steam cracking the amount of C.sub.5 products has been minimized, although at a correspondingly higher concentration of lower carbon atom number foulant precursors such as di-olefinic, poly-olefinic and acetylinic compounds. This development has served to exacerbate the fouling problem which has heretofore been encountered in the fractional distillation of C.sub.2, C.sub.3 and C.sub.4 fractions from each other and from heavier hydrocarbons. Fouling of the debutanizer unit by reason of the aforementioned increase in the concentration of foulant precursors has become a particular problem of increased concern.
A considerable amount of work has been done on improving the basic process of separating the products of steam cracking. Much of the work on light ends fractionation has been concerned with the improvement of the various components of the process. Other improvements relate to improved computer control of the process. Progress has also been made in the optimum design and operation of the process through the use of improved physical property correlations. Although there have been improvements in the sophistication of the design of fractionation steps such as two-tower demethanizers, deethanizers, and depropanizers, heat-pumped towers, and improved separation efficiencies through the use of dephlegmators, the basic flow sequences have remained essentially unchanged.
One of the basic problems encountered in such fractional distillation processes relates to polymer fouling of the fractional distillation columns. One such problem, for example, relates to the production of foulant precursors in steam cracking which at high temperatures cause fouling in equipment. It is well known that the rate of polymer fouling increases as temperature increases. Such fouling often necessitates the shutdown of the distillation unit for cleaning. Both the shutdown and cleaning involve significant expense.
While changes in the operating conditions, as in for example reducing the operating temperature and/or pressure, have been used to control these fouling problems, such changes are many times not sufficient to overcome the problem completely. In addition, operating modifications can result in reduced production efficiency which translates to an associated decrease in revenues.
U.S. Pat. No. 4,545,895 to Brand et. al. teaches yet another alternative approach to reduce fouling. This process relates to the reduction of fouling by controlling reboiler temperatures.
U.S. patent No. 3,783,126 to Hayward et. al. teaches a dual pressure fractionation tower with a high pressure and a low pressure section. The process of the invention requires delivery of the overhead vapors from the low-pressure column to the high pressure column. The invention is particularly suitable for use as a depropanizer.
U.S. Pat. No. 4,824,527 to Erickson teaches a method of fractionating liquid mixtures of unequal amounts of heavy and light product fractions wherein two columns are used. In the case of a feed mixture having a majority of light product the heavy feed from a rectifier column is delivered into the second column.
U.S. Pat. No. 4,002,554 to Borge et. al. teaches an approach to minimize fouling in the internal surface of a metallic heating unit involving the purging of the heating unit with an inert gas followed by introduction of nitric oxide into said heating unit. The heating unit is subsequently purged with an inert gas whereupon the heating unit is ready for introduction of a hydrocarbon feedstock.
U.S. Pat. No. 4,670,131 to Ferrell teaches an alternative approach to minimize fouling. This process relates to inhibition of polymerization of olefinic compounds which results in fouling by introduction of stable free radicals, such as nitroxide, into the system.
A need still exists for a method of reducing fouling which does not require the addition of extraneous chemicals which could affect quality of the final product, since these extraneous chemicals are expensive and do not fully control fouling.